In many imaging applications, the object to be imaged includes a highly remittive layer. When light illuminates such an object, the resulting image consists of a directly scattered component reflected from this highly remittive layer and a multiply scattered component which is scattered from points that are within the object and outside the highly remittive layer. Because the layer is highly remittive, the directly scattered component tends to dominate the image. As a result, it is difficult to capture the multiply scattered component of the image.
An example of an object having a highly remittive layer is the human retina. In the retina, certain structures are visible only by examination of the directly scattered component of the image. These structures cannot be seen clearly by examination of the multiply scattered component. Examples of such structures include small blood vessels and superficial features of the optic nerve head. Conversely, there exist other retinal structures, such as drusen and sub-retinal edema, which are visible to a far greater extent in the multiply scattered component. Some of these structures cannot readily be observed by examination of the directly scattered light. Accordingly, it is desirable to provide an ocular fundus imaging apparatus for permitting an eye-care specialist to switch easily between observation of the directly scattered component and observation of the multiply scattered component of the image.
A known technique for separating an image into its multiply scattered component and its directly scattered component is to illuminate the retina with a point light source and to direct the remitted image field through a field stop confocal to the light source. By providing the field stop with a pinhole aperture, one can observe the directly scattered component of the image. Alternatively, by providing the field stop with an annular opening, one can observe the multiply scattered component of the image. These techniques are described in Elsner A. E., Burns S. A., Weiter J. J., and Delori F. C., Infrared imaging of subretinal structures in the human ocular fundus, Vision Research 36, 191-205, 1996.
Using the foregoing technique, one can provide a field stop with a pinhole aperture, observe the directly scattered component of the image, replace the pinhole aperture with an annular aperture, and then proceed to observe the multiply scattered component of the image. By scanning in two dimensions, one can then generate a two-dimensional image which includes only the multiply scattered component and create another image which includes only the directly scattered component. Similarly, by using known techniques of tomography, one can obtain pairs of cross sections, one including only the multiply scattered component and another which includes only the directly scattered component.
In certain opthalmological applications, it is desirable to precisely locate a structure which can be imaged in the multiply scattered component with respect to a known feature observable only in the directly scattered component. For example, it may be useful to know that a particular region of drusen or edema is located near the intersection of two blood vessels.
A disadvantage of the foregoing known technique is that a significant interval elapses between the measurement of the directly scattered component and the subsequent measurement of the multiply scattered component. This interval arises because of the time required to replace the pinhole aperture with an annular aperture.
Using the method described above, one can, in principle, accomplish the task of precisely locating a structure visible in one component relative to a feature visible in the other component by capturing an image of the directly scattered field and then overlaying it on the image of the multiply scattered field. By aligning the image from the multiply scattered component with the image from the directly scattered component, one can then locate a structure visible only in one component relative to a structure visible only in the other component.
In practice, however, the retina is constantly subject to rapid and unpredictable motion. As a result, in the brief interval, referred to as a blanking interval, that elapses as the annular aperture replaces the pinhole aperture, the retina will have moved by some unknown amount. Since a patient cannot eliminate eye movements, the position of the retina during observation of the multiply scattered component will, in general, not be the same as the position of the retina during observation of the directly scattered component. This unpredictable motion of the retina interferes with the reliable alignment of the two images.
The foregoing disadvantage can, in principle, be mitigated by reducing the blanking interval. If the blanking interval is made short enough, the retina will move a negligible amount between the observation of the directly scattered component and the observation of the multiply scattered component.
In practice, however, the mechanical inertia associated with replacing the pinhole aperture with an annular aperture prevents the blanking interval from being made short enough to capture two successive images without noticeable movement of the retina between images. What is necessary and desirable in the art therefore is an apparatus and method for reducing the blanking interval, thereby permitting observation of the directly scattered component and the multiply scattered component of an image field substantially simultaneously.